1. Field of the Invention
The present invention relates generally to a radar apparatus to measure the distance to a target object and the relative speed thereto, a signal processing method and a program, and more particularly to a radar apparatus capable of accurately measuring a plurality of objects that are present in proximity, a signal processing method and a program.
2. Description of the Related Arts
In recent years, an ACC (Adaptive Cruise Control) system to control the accelerator or brake of a vehicle running on an expressway has being thought about, in the field of ITS (Intelligent Transport Systems). What is indispensable to this ACC system is a sensor to measure the distance of the vehicle running in the front and relative speed. As this sensor, millimeter wave radar of a FM-CW (Frequency-Modulated Continuous Waves) type holds great promise. The FM-CW millimeter wave radar generates beat signal from the difference between the frequency modulated sending wave and wave received from the target object (hereinafter to be referred to as xe2x80x9ctargetxe2x80x9d), performs FFT processing (Fast Fourier Transform processing), separating the rise zone and the fall zone of FM frequency, and extracts individual beat frequency of both of the two zones to be peak level. And from the two beat frequencies extracted from the individual zones, the radar determines beat frequency of the target by pairing processing, so as to find the distance and relative speed of the target. Also, as the distance and relative speed of the target can be acquired for every antenna angle, when a radar antenna is allowed to scan in a horizontal direction, the target can be detected two-dimensionally.
However, the FM-CW millimeter wave radar has a problem that the separation of a plurality of targets in proximity would be difficult. In order to separate targets in distance direction, frequency contained in beat signal must be separated and extracted. When a plurality of targets present in proximity, a plurality of signals may appear in nearby beat frequency. To separate a plurality of beat frequencies, the separation can be made with a condition that there is a valley between peak-to-peak of the frequency spectrum acquired by the FFT processing. However, when beat frequencies are in close positions, as peaks of the frequency spectrum are synthesized, overlaid a peak on another peak, peaks cannot be separated. The limitation of separating peaks is determined depending on the number of points of the FFT or window function, but especially at the boundary between availability and unavailability of separation, the boundary between availability and unavailability can become unstable caused by phase difference of signals. When difference in frequencies of a plurality of beat signals becomes the boundary between availability and unavailability of separation, the reflected waves from a plurality of targets cause a synthesizing method of signals to change corresponding to the difference in the individual phases, and the level of the valley between peak-to-peak significantly changes. Therefore, the boundary between availability and unavailability of peak separation can change depending on phase difference, thereby leading up to unstable separation or likely resulting in erroneous pairing to pair beat frequencies of different targets together. Similarly, when the antenna is scanning in a horizontal direction, there is a problem that can lead to erroneous pairing, if an attempt is made to separate targets located at the same distance and at nearby angles, because the boundary between availability and unavailability of separation would change depending on the difference in the individual phases.
According to the present invention there are provided a radar apparatus, a signal processing method and a program, ensuring that the separation of a plurality of targets present in proximity is made more securely.
(Antenna Non-Scanning Type Radar Apparatus)
A first aspect of the present invention provides a radar apparatus comprising a sending/receiving unit which sends from its antenna a sending signal which has been frequency modulated (FM) by a triangular wave signal, the sending/receiving unit mixing a signal received from the antenna and a local signal branched off from the sending signal, to thereby generate a beat signal; a frequency analyzing unit which analyzes (by FFT) the frequency of the beat signal for each rise zone and each fall zone of the frequency modulated by the triangular wave signal; an accumulation unit which accumulates a plurality of frequency spectra by frequency analysis at least for each the rise zone; a judgment unit which judges a valley (characteristic section) from the plurality of frequency spectra accumulated in the accumulation unit; a peak frequency extraction unit which acquires, as beat frequencies of different targets, respective beat frequencies of peak sections located at both sides of the characteristic section judged by the judgment unit; and an operation unit which calculates the distance to a target and the relative speed thereto, based on the beat frequencies.
According to considerations of the inventor of the present invention, each of the reflected waves from a plurality of targets has different phase, and if the radar or the target is traveling, phase will change at random, in the light of the wavelength of millimeter wave. Also, as to the reflected waves from the plurality of targets, the synthesizing method of signals would change depending on the individual phase difference, so, if beat frequencies to form a plurality of peaks are close, the level between peak-to-peak will change considerably. Therefore, the present invention is intending to perform the separation of targets present in proximity more securely, accumulating the distribution of frequency spectrum, that is the result of analyzing frequencies where the overlapping state of peak-to-peak changes caused by phase changing at random, performing the accumulation work a plurality of times repeatedly in time sequence, and from these accumulated distributions, judging the frequency of dropping sections with large level difference to be a featuring section, and setting each of two peaks at the front and back of the featuring section as beat frequency of the target. Here, an accumulation unit, judgment unit and a peak frequency extraction unit can be designed to perform processing of both of the FM rise zone and FM fall zone, so as to further make sure the separation of targets.
A second aspect of the present invention provides a signal processing method for the FM=CW radar. The method comprises a sending/receiving step which includes sending from an antenna a sending signal which has been frequency modulated by a triangular wave signal, and mixing a signal received from the antenna and a local signal branched off from the sending signal, to thereby generate a beat signal; a frequency analysis step which includes analyzing (by FFT) the frequency of the beat signal for each rise zone and each fall zone of the frequency modulated by the triangular wave signal; an accumulation step which includes accumulating a plurality of frequency spectra by analyzing the frequency at least for each the rise zone; a judgment step which includes judging a characteristic section from the plurality of frequency spectra accumulated; a peak frequency extraction step which includes acquiring, as beat frequencies of different targets, respective beat frequencies of peak sections located at the both sides of the characteristic section judged; and an operation step which includes calculating the distance to a target and the relative speed thereto, based on the beat frequencies. Here, the accumulation step, judgment step and the peak frequency extraction step may be executed as to both of the rise zone and the fall zone.
A third aspect of the present invention provides a program executed by a computer (e.g., DSP) mounted on the radar apparatus. The program causes a computer mounted on a radar apparatus to execute a frequency analysis step which includes inputting a beat signal acquired by sending from an antenna a sending signal which has been frequency modulated by a triangular wave signal, and by mixing a signal received from the antenna and a local signal branched off from the sending signal, and analyzing (by FFT) the frequency of the beat signal for each rise zone and each fall zone of the frequency modulated by the triangular wave signal; an accumulation step which includes accumulating a plurality of frequency spectra by analyzing the frequency at least for each the rise zone; a judgment step which includes judging a characteristic section from the plurality of frequency spectra accumulated; a peak frequency extraction step which includes acquiring, as beat frequencies of different targets, respective beat frequencies of peak sections located at the both sides of the characteristic section judged; and an operation step which includes calculating the distance to a target and the relative speed thereto, based on the beat frequencies.
(Antenna Scanning Type Radar Apparatus)
According to considerations of the inventor of the present invention, in the radar apparatus scanning an antenna in a horizontal direction, when a plurality of targets at the same distance are located at near angles viewed from the antenna, there is the angular range where availability or unavailability of separation would be unstable depending on phase differences of a plurality of received waves, and phase would change at random, if the radar or the target is traveling.
Thus, a fourth aspect of the present invention provides a radar apparatus comprising an antenna which is mechanically scanned within a predetermined angular range; a sending/receiving unit which sends from the antenna a sending signal which has been frequency modulated by a sending triangular wave signal, the sending/receiving unit mixing a signal received from the antenna and a local signal branched off from the sending signal, to thereby generate a beat signal; a frequency analyzing unit which analyzes the frequency of the beat signal for each rise zone and each fall zone of the frequency modulated by the triangular wave signal; an accumulation unit which accumulates a plurality of changes in spectral value of a specific beat frequency acquired by the frequency analyzing unit, corresponding to changes in the antenna scanning angle, for each different antenna scanning angle within a predetermined angular range; a judgment unit which judges a characteristic section from changes in spectral value corresponding to a plurality of antenna scanning angles accumulated in the accumulation unit; a peak frequency extraction unit which acquires, as beat frequencies of targets present in different directions of scanning angle, spectral values of peak sections located at both sides of the characteristic section judged by the judgment unit; and an operation unit which calculates the distance to a target and the relative speed thereto correlated with the antenna angle, based on the beat frequencies. In this manner, as to the spectral value of the beat frequencies extracted from a plurality of targets located at the same distance and in different directions, the present invention accumulates distributions showing changes in the peak-to-peak overlapping state in the directions of the individual target caused by phase changing at random, a plurality of times in time sequence, and judges the frequency of falling section from these, where level difference is large, as a valley section. And, the present invention intends to separate a plurality of targets located at the same distance and in proximity more securely, recognizing two peak directions at the front and back of the valley section as individual targets.
Herein, the antenna scanning type radar apparatus performs the processing at a specified antenna scanning position, with its frequency analyzing unit further including a second accumulation unit which accumulates a plurality of frequency spectra by analyzing the frequency at least for each the rise zone; a second judgment unit which judges a characteristic section from the plurality of frequency spectra accumulated in the second accumulation unit; and a second peak frequency extraction unit which acquires, as beat frequencies of different targets, respective beat frequencies of peak sections located at both sides of the characteristic section judged by the second judgment unit. The second accumulation unit, the second judgment unit and the second peak frequency extraction unit may process both the rise zone and the fall zone.
A fifth aspect of the present invention provides a signal processing method of an FM-CW radar. The signal processing method comprises a sending/receiving step which includes sending a sending signal which has been frequency modulated by a triangular wave signal, from an antenna which is mechanically scanned within a predetermined angular range, and mixing a signal received from the antenna and a local signal branched off from the sending signal, to thereby generate a beat signal; a frequency analysis step which includes analyzing the frequency of the beat signal for each rise zone and each fall zone of the frequency modulated by the triangular wave signal; an accumulation step which includes accumulating a plurality of changes in spectral value of a specific beat frequency acquired by the frequency analysis, corresponding to changes in the antenna scanning angle, for each different antenna scanning angle within a predetermined angular range; a judgment step which includes judging a characteristic section from changes in spectral value corresponding to a plurality of antenna scanning angles accumulated; a peak frequency extraction step which includes acquiring, as beat frequencies of targets present in different directions of scanning angle, spectral values of peak sections located at both sides of the characteristic section judged; and an operation step which includes which calculates the distance to a target and the relative speed thereto correlated with the antenna angle, based on the beat frequencies.
Herein, the frequency analysis step further includes a second accumulation step which includes accumulating a plurality of frequency spectra by analyzing the frequency at least for each the rise zone; a second judgment step which includes judging a characteristic section from the plurality of frequency spectra accumulated; and a second peak frequency extraction unit which acquires, as beat frequencies of different targets, respective beat frequencies of peak sections located at both sides of the characteristic section judged.
The second accumulation step, the second judgment step and the second peak frequency extraction step may include processing both the rise zone and the fall zone.
A sixth aspect of the present invention provides a program executed by a computer (DSP) mounted on an FM-CW radar. The program causes the computer to execute a frequency analysis step which includes inputting a beat signal acquired by sending a sending signal which has been frequency modulated by a triangular wave signal, from an antenna which is mechanically scanned within a predetermined angular range, and by mixing a signal received from the antenna and a local signal branched off from the sending signal, and analyzing the frequency of the beat signal for each rise zone and each fall zone of the frequency modulated by the triangular wave signal; an accumulation step which includes accumulating a plurality of changes in spectral value of a specific beat frequency acquired by the frequency analysis, corresponding to changes in the antenna scanning angle, for each different antenna scanning angle within a predetermined angular range; a judgment step which includes judging a characteristic section from changes in spectral value corresponding to a plurality of antenna scanning angles accumulated; a peak frequency extraction step which includes acquiring, as beat frequencies of targets present in different directions of scanning angle, spectral values of peak sections located at both sides of the characteristic section judged; and an operation step which includes which calculates the distance to a target and the relative speed thereto correlated with the antenna angle, based on the beat frequencies.
(Fixedly Installed Type Radar Apparatus)
A seventh aspect of the present invention provides a radar apparatus which is fixedly installed in a road structure for, e.g. monitoring the traffic. The radar apparatus comprises an antenna securely installed on a structure; a sending/receiving unit which sends from its antenna a sending signal which has been frequency modulated by a triangular wave signal, the sending/receiving unit mixing a signal received from the antenna and a local signal branched off from the sending signal, to thereby generate a beat signal; a frequency analyzing unit which analyzes the frequency of the beat signal for each rise zone and each fall zone of the frequency modulated by the triangular wave signal; an accumulation unit which accumulates a plurality of frequency spectra by frequency analysis at least for each the rise zone; a judgment unit which judges a characteristic section from the plurality of frequency spectra accumulated in the accumulation unit; a peak frequency extraction unit which acquires, as beat frequencies of different targets, respective beat frequencies of peak sections located at both sides of the characteristic section judged by the judgment unit; an operation unit which calculates the distance to a target and the relative speed thereto, based on the beat frequencies; and an antenna drive unit which periodically moves the installation position of the antenna within a specified range conforming to the wavelength used. In the fixedly installed type radar apparatus, phase of the received wave will not change at random, unlike the radar apparatus mounted on a vehicle or other mobile bodies, but phase will be fixed. So, the present invention intends to securely separate a plurality of targets present in proximity in a specified antenna direction, forcibly creating phase change at random, only by moving the position of the antenna. Herein, the accumulation unit, the judgment unit and the peak frequency extraction unit may process both the rise zone and the fall zone.
An eighth aspect of the present invention provides a signal processing method of a scanning type radar apparatus. The signal processing method comprises an antenna drive step which periodically moves the installation position of an antenna which is fixedly installed in a structure, within a minute range corresponding to the wavelength used; a sending/receiving step which includes sending from the antenna a sending signal which has been frequency modulated by a triangular wave signal, and mixing a signal received from the antenna and a local signal branched off from the sending signal, to thereby generate a beat signal; a frequency analyzing step which includes analyzing (by FFT) the frequency of the beat signal for each rise zone and each fall zone of the frequency modulated by the triangular wave signal; an accumulation step which includes accumulating a plurality of frequency spectra by frequency analysis at least for each FM rise zone; a judgment step which includes judging a characteristic section from the plurality of frequency spectra accumulated; a peak frequency extraction step which includes acquiring, as beat frequencies of different targets, respective beat frequencies of peak sections located at both sides of the characteristic section judged; and an operation step which includes calculating the distance to a target and the relative speed thereto, based on the beat frequencies. Herein, the accumulation step, the judgment step and the peak frequency extraction step may be effected of both the FM rise zone and the FM fall zone.
The above and other objects, aspects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjugation with the accompanying drawings.